Tablets can be classified into two major categories, i.e. (i) an uncoated tablet prepared by compression-molding fine granules, granules or a mixed powder and (ii) a coated tablet prepared by coating an uncoated tablet with a film or sugar. When the uncoated tablet has a curved surface, it may have problems with moldability. Thus, the uncoated tablet is usually molded into a flat form in order to insure productivity and handling efficiency.
Since the coated tablet has a smooth and mechanically strong surface, no powder is broken out due to wear or abrasion. Therefore, the coated tablet has advantages that it can be employed in a visual examination machine with a high handling and packaging efficiency, as well as in an automatic compounding machine in a hospital or pharmacy with an excellent handling efficiency. The coating layer of the coated tablet, however, suppresses disintegration of the tablet and dissolution of an active ingredient, thus it is unsuitable for a pharmaceutical preparation wherein fast disintegration or dissolution is required. Further, the coated tablet has disadvantages of being high costed owing to a large number of production steps.
On the contrary, though the above-mentioned uncoated tablet can be advantageously produced with ease and simplicity and with a low production cost, the surface thereof is worn or abraded in preparation or transportation process, and powder thus produced is attached or affixed to the surface of the uncoated tablet or remained in a package. Thus, the commercial value of the products is significantly reduced. Further, because of the powder attached or affixed to the tablet, the working efficiency in a visual examination machine and packaging efficiency may be reduced. Furthermore, the uncoated tablet also confronts with a lower working efficiency in an automatic compounding machine.
Regarding the techniques to suppress such development of powder due to wearing and abrading of an uncoated tablet, Japanese Patent Publication No. (JP-B) 25926/1992 discloses a coating method which comprises preparing a suspension or dispersion containing a higher fatty acid ester having a high melting point of 70.degree. C. or more, a cellulose derivative soluble in a hydrophilic solvent and the hydrophilic solvent and coating the surface of an uncoated tablet with said suspension or dispersion in a proportion of 0.02 to 0.8% by weight on the dried basis.
This technique, however, requires a number of steps, that is, a step for preparing beforehand uncoated tablets by compression-molding, a step for preparing said suspension or dispersion by emulsifying or dispersing said higher fatty acid ester in a hydrophilic solvent using, if necessary, a surfactant, and a coating step for coating the tablets with said suspension or dispersion. Further, since the technique is a kind of coating techniques for coating the surface of tablets, the producing steps are complicated and an additional equipment for coating is further required to be arranged in the producing line of tablets.
It should be generally understood, as described in the above-mentioned literature, that even when a composition containing a lubricant is compressed and molded into a tablet, uncoated tablets having a practically satisfied abrasion resistance can not be obtained.
European Patent Application No. EP-546358 A2 discloses a stabilized pharmaceutical composition for oral use which is obtained by compression-molding a composition comprising the benzimidazole compound and an oily substance having a lower melting point, in order to suppressing decomposition of a benzimidazole compound as an active ingredient due to deformation of crystals caused by, for example, pressure.
U.S. Pat. No. 5,055,304 corresponding to Japanese Patent Application Laid-open No. (JP-A) 308231/1989 discloses a stabilized pharmaceutical composition which is obtained by compressing and molding a composition comprising disodium adenosine triphosphate and an oily substance having a lower melting point in order to suppressing the decomposition of disodium adenosine triphosphate.
In these literatures, said oily substances having lower melting points are used for stabilizing the active ingredients, and improvements in an abrasion resistance of an uncoated tablet were never taught by means of adding an oily substance having a lower melting point.